fix freebsd compilation error

[yjqg6666]

I have compilation error on FreeBSD 14.1 when run pip install mitmproxy. mitmproxy depends on mitmproxy-rs which depends on boringtun.

OS: FreeBSD 14.1-RELEASE-p5
Arch: amd64
Rustc: rustc 1.81.0 (f54dd915b 2024-09-02)

Compiling boringtun v0.6.0 (https://github.com/cloudflare/boringtun?rev=e3252d9c4f4c8fc628995330f45369effd4660a1#e3252d9c)
error[E0599]: no associated item named CLOCK_BOOTTIME found for struct ClockId in the current scope
--> /home/user/.cargo/git/checkouts/boringtun-379ccad4d4acd372/e3252d9/boringtun/src/sleepyinstant/unix.rs:9:36
9 | const CLOCK_ID: ClockId = ClockId::CLOCK_BOOTTIME;
| ^^^^^^^^^^^^^^ associated item not found in ClockId
|
help: there is an associated constant CLOCK_UPTIME with a similar name
|
9 | const CLOCK_ID: ClockId = ClockId::CLOCK_UPTIME;

Related issue: mitmproxy/mitmproxy#6378, #386 #29.

[yjqg6666]

Another info: Redefine CLOCK_BOOTTIME to alias CLOCK_MONOTONIC, not CLOCK_UPTIME